The conventional rotation angle detector has an incremental signal encoder for detecting an absolute steer angle (e.g., 720° when steered in the right direction) of a steering wheel that makes multiple rotations. As shown in FIG. 1, an incremental signal encoder 100 includes a rotating plate 40 connected to a steering shaft ST and optical sensor arrays 41 for detecting a slit array (not shown) provided for the rotating plate 40.
When the rotating plate 40 rotates due to steering of the steering wheel, the encoder 100 sequentially detects each slit of the slit array and produces an associated pulse signal that corresponds to each sensed slit. The pulse signal is supplied to an electronic control unit (not shown). The encoder 100 produces an original signal based on a reference slit that is detected when the steer angle is 0° and supplies the original signal to the electronic control unit.
The electronic control unit counts the pulse number of the pulse signals sequentially received based on the steer angle amount of the steering wheel after the original signal is initially received. The electronic control unit detects the absolute steer angle of the steering wheel in accordance with the count value of the pulse number.
However, in the case of detecting the absolute steer angle of the steering wheel with the incremental signal encoder, when the power supply to the electronic control unit is stopped due to switching the ignition switch of the vehicle off, the absolute steer angles calculated up to then are lost. Therefore, when the ignition switch is switched on again and the power supply for the electronic control unit is started, the encoder 100 must be returned to zero again. Therefore, when the ignition switch is switched on again, the steer angle cannot immediately be detected.
Therefore, in the case of detecting the absolute steer angle with the incremental signal encoder, if the power source is still supplied to the electronic control unit after the ignition switch is switched off, the absolute steer angles calculated up to then are maintained. In this case, the power consumption increases since the power source must be supplied to the electronic control unit while the ignition switch is switched off.
To immediately detect the steer angle without increasing power consumption when the ignition switch is switched on again, for example, an absolute signal encoder 200 shown in FIG. 2 may be used for detecting the absolute steer angle.
The absolute signal encoder 200 detects the absolute steer angle of the steering wheel that makes multiple rotations and includes a first encoder 70 connected to a steering shaft ST and a second encoder 71 connected to the first encoder 70.
When the absolute steer angle is detected by only one encoder actuatingly connected so that the encoder makes one complete rotation with respect to the whole steering range of the steering shaft ST, the resolution is degraded. Therefore, the absolute signal encoder 200 includes two encoders 70 and 71.
The first encoder 70 includes a rotating plate 72 connected to the steering shaft ST and the rotating plate 72 includes a gear 73. The first encoder 70 detects in the range of one complete rotation of the steering wheel to obtain the absolute rotation angle of the steering shaft ST. The second encoder 71 includes, for example, a gear 75 fixed to an input shaft 74. The gear 75 meshes with the gear 73 of the rotating plate 72. While the rotating plate 72 makes two complete rotations, the input shaft 74 makes one complete rotation.
The second encoder 71 detects in the range of two complete rotations of the steering wheel to obtain the absolute rotation angle of the steering shaft ST. That is, the first encoder 70 detects the absolute steer angle of the steering wheel through one complete rotation with high resolution. The second encoder 71 detects the absolute steer angle of the steering wheel throughout all of the steering range with lower resolution than that of the first encoder 70. As a result, the absolute steer angle of the steering wheel throughout all steering ranges can be detected with high resolution.
In the encoder 200 including two absolute signal encoders 70, 71, it is not necessary to supply the power source to the electronic control unit while the ignition switch is switched off and the absolute steer angle may be detected with high resolution.
However, when the absolute steer angle is detected by the two encoders 70 and 71, the rotation speed of the input shaft 74 of the second encoder 71 must be reduced to one-half of that of the steering shaft ST. Therefore, the outer diameter of the gear 75, which is fixed to the input shaft 74, is larger than that of the steering shaft ST. This increases the space occupied by the encoder 200 (a rotation angle detector) with respect to the size of the steering shaft ST. As a result, the installation of the rotation angle detector in a vehicle becomes difficult. The difficulty of the installation becomes increasingly pronounced as the rotation range of the steering shaft ST increases.